A typical example of a circuit in accordance with the prior art is a color demodulator circuit in a color television receiver. This circuit includes three circuit portions which respectively demodulate color signals of red, blue and green components. The demodulator circuit further includes constant current sources, and each of the demodulating circuit portions is connected to the respective constant current sources. Each demodulating circuit portion has an output terminal from which a demodulated color signal is derived, and the D.C. potential at the output terminals of the three demodulating circuit portions is required to be the same so that highly accurate color reproduction, notably reproduction of white colors, can be secured on a screen of the color television receiver. This requirement must be met, in the prior art, by controlling relative values of the output currents of the respective constant current sources. However, it is quite difficult to accurately control the relative values of the output currents of prior art constant current sources, as is detailed hereinafter.
The relative values of the output currents, in the prior art, are determined by the value of the current gain .alpha., or its factorial, of the transistor or transistors in the respective circuit portions connected to the constant current sources. However, since the current gain .alpha. is 0.98-0.99, i.e., nearly equal to unity, accurate control of the relative output current values is very hard and is not suited to mass-production techniques. This is especially true where the circuit is realized as a semiconductor integrated circuit. In such a circuit reliziation, accurate control is impossible. In addition, even if accurate control of the relative values of the output currents can be achieved for one specific value of the current gain .alpha., the controlled relationship of the output D.C. potentials will be lost with variations of the current gain .alpha. due to operating temperature fluctuation or manufacturing error.
Difficulties similar to those just described will also occur in the circuit of a direct-coupled amplifier, direct-coupled modulator, direct-coupled demodulator or other circuits which generate two or more outputs and require the accurately controlled output D.C. potentials.
It is therefore a principle object of the present invention to provide a transistor circuit generating at least two output potential having accurately controlled D.C. currents.
It is another object of the present invention to provide a transistor circuit, having two or more constant current sources and generating two or more output potentials, having D.C. currents, the relation of which, is maintained irrespective of the current gain .alpha. of transistors employed.
It is still another object of the present invention to provide a transistor circuit including two or more circuit portions, each connected to a respective constant current source and generating outputs having the same D.C. potential irrespective of the current gain .alpha. of the transistor employed in the circuit portion.
According to the present invention, there is provided a transistor circuit comprising:
a power supply terminal; PA1 a reference constant voltage supply terminal; PA1 a first constant current source having a first transistor with a base connected to said reference constant voltage supply terminal, an emitter connected to a common potential by way of a first resistor, and a collector generating a first constant current; PA1 a first circuit portion including a series connection of a third transistor connected to the collector of the first transistor and a first resistive load connected to the power supply terminal; PA1 a second constant current source having a second transistor with an emitter connected to the common potential by way of a second resistor, a collector generating a second constant current, and a base connected by way of a third resistor to said reference voltage supply terminal; and PA1 a second circuit portion including a second resistive load connected between the collector of the second transistor and the power supply terminal.
It is a feature of the invention that a compensating resistor is inserted between the base of the transistor of one of the constant current sources and the reference voltage supply terminal.
In accordance with the invention, one or more transistors may be inserted in a series-connection relationship between the thid transistor of the first circuit portion and the collector of the first transistor. Further, one or more series-connected transistors may be inserted between the second resistive load of the second circuit portion and the collector of the second transistor, provided that the number of the inserted transistor or transistors is less than that of the series-connected transistors in the first circuit portion. The resistance of the compensating resistor is determined so as to equalize the variation of the currents flowing through the first and second resistive loads. Such variation is caused by operating temperature fluctuation or manufacturing error.
It is another feature of the invention that in the case where the difference "K" in the number of transistors between the first and second circuit portions, is equal to 3, the resistance of the compensating resistor is selected so as to be "K" times as large as the resistance of the second resistor.
According to another aspect of the present invention, as will be described in more detail hereinafter, D.C. currents flowing through the first and second resistive loads may be equally affected from the first and second circuit portions irrespective of the difference in numbers of the series-connected transistors in these circuit portions. Therefore the relation of these currents is kept uniform irrespective of the variation of the current gain .alpha. of the transistor due to temperature fluctuation or manufacturing error.
It is a further feature of the invention that in the case where the resistances of the first and second resistors in the first and second constant current sources are designed to be equal to each other and where the resistances of the first and second resistive loads in the first and second circuits are designed to be equal to each other, the same D.C. voltage drop appears across the first and the second resistive loads irrespective of the variation of the current gain .alpha. due to temperature fluctuation or manufacturing error.
It is another feature of the invention that the resistance of the compensating resistor may be set to be an integral number of times as large as that of the second resistor, so that the circuit arrangement may be simplified. Although difficulty is encountered with adjustment of an absolute value, of a resistance in a semiconductor integrated circuit, it is quite easy to multiply a ratio of resistance an integral number of times, so that the present invention may be readily practiced in a semiconductor integrated circuit.